The proposal that helper T-cells recognise a foreign material, e.g. a viral antigen, when it is presented on the surface of an antigen-presenting cell as a complex with the major histocompatibility complex (MHC) class II glycoprotein. This is contrasted with a less favoured explanation, the intimacy or dual-recognition model, which postulates that helper T-cells must simultaneously recognise both the foreign antigen and the separate MHC complex. By a simple extension, the altered-self hypothesis further proposes that a chemically altered MHC may also be recognised by helper T-cells.